The present invention relates generally to an electronic control system for controlling operation of an internal combustion engine. More specifically, the invention relates to an engine control system which detects specific engine operating conditions under which engine stall may occur and performs a back-up operation to prevent the engine from stalling.
SAE Papers 800056 and 800825, published by Society of Automotive Engineers discloses electronic control systems for internal combustion engines for controlling fuel supply, fuel injection, auxiliary air flow, spark ignition, exhaust gas recirculation and so forth according to predetermined engine control parameters. Control may be performed in closed loops and/or open loops to derive control signals for each of the enging operating elements controlled depending upon the engine operating conditions. In such control systems, the engine operating conditions to be detected have already occurred some time before they are actually detected. Response lags occur in the control system as well as in the element to be controlled. Such lags may be significant when the engine is under critical conditions.
Numerous experiences of engine stalling under certain driving conditions have been reported such as under relatively heavy load conditions while driving the compressor of an air conditioner, the alternator, the radiator fan and so forth. In modern vehicles, the load on the engine tends to be increased by installation of power steering which requires an engine-driven pump, air-conditioning which requires a compressor driven by the engine, a relatively high-capacity alternator for generating electric power at high ratings, and so forth. Furthermore, increases in the electrically operated accessories such as automotive audio systems, high-capacity blowers for the air conditioner, and so forth, affect engine operation by lowering the supply voltage for an ignition system which may cause engine stalling.
An engine stall preventive engine control system has been proposed in Published Japanese Patent (Tokko) No. Showa 49-40886, published on Nov. 6, 1974. In the disclosed system, actual engine speed is compared with a predetermined threshold. When the engine speed drops below the threshold, a stall-preventing operation is performed. In the stall-preventing operation, an auxiliary air flow rate is increased and/or the fuel supply or fuel injection quantity is increased to increase engine output torque.
However, in the control system of the above-mentioned Published Japanese patent, excessive time lags, which may prevent successful execution of the engine stall-preventing operation, exist due to the nature of the engine itself. For instance, after a control signal is issued to increase the auxiliary air flow rate, the auxiliary air control valve is actuated so as to allow an increased rate of air flow, but only after a certain time lag. The increase in the of auxiliary air flow rate is recognized only after another time lag. After another time lag, the fuel is increased. Finally, engine torque increases to a sufficient level to prevent the engine from stalling. However, the accumulated time lag may be sufficient to allow the engine to stall due to response delays.
In addition, in the aforementioned stall preventing operation, engine operation fluctuates significantly due to response delays in increasing the air flow rate and fuel supply amount and due to significant deviation of air/fuel ratio from the stoichiometric value. This further prevents successful stall prevention.